In a radio communication system, transmission diversity may be used on the base station side in order to increase reception power of transmission signals of individual channels (hereinafter referred to as “individual channel signals”) on the communication terminal side by sending individual channel signals to one communication terminal from a plurality of diversity antennas.
FIG. 1 is a system configuration diagram of a system disclosed in the 3GPP WG1 TSG-RAN WG1 R1-99832 (Physical channels and mapping of transport channels onto physical channels (FDD)), as one example of a radio communication system using transmission diversity.
As shown in FIG. 1, base station 1 transmits common pilot channel transmission signal (hereinafter referred to as “common pilot channel signal”) A from antenna A and transmits common pilot channel signal B from antenna B. At the same time, base station 1 transmits individual channel signal A to communication terminal 2 from antenna A and transmits individual channel signal B to communication terminal 2 from antenna B.
Since individual channel signal A and individual channel signal B are multiplied by a same spreading code at base station 1, communication terminal 2 receives individual channel signal A and individual channel signal B as one inseparable signal.
On the other hand, common pilot channel signal A and common pilot channel signal B are multiplied by different spreading codes. Or even if these two signals are multiplied by a same spreading code, these are made separable in some way. Therefore, communication terminal 2 can separate common pilot channel signal A from common pilot channel signal B. Moreover, individual channel signal A and common pilot channel signal A, and individual channel signal B and common pilot channel signal B are received through a same propagation path respectively, and therefore it is possible to know a phase rotation angle of individual channel signal B with respect to individual channel signal A by carrying out channel estimations of common pilot channel signal A and common pilot channel signal B.
FIG. 2 is a block diagram showing a configuration of a conventional communication terminal. In the communication terminal shown in FIG. 2, antenna 11 receives a signal transmitted from a base station and sends a signal to the base station. Duplexer 12 switches between time zones of transmission and reception. Reception RF section 13 amplifies the reception signal that passes duplexer 12 and converts the frequency of the reception signal to a baseband signal.
Despreading section 14 despreads the output signal of reception RF section 13 with a spreading code of an individual channel signal and extracts a modulated signal of an individual channel signal. Likewise, despreading section 15 despreads the output signal of reception RF section 13 with a spreading code of a common pilot channel signal A and extracts a modulated signal of common pilot channel signal A. Likewise, despreading section 16 despreads the output signal of reception RF section 13 with a spreading code of a common pilot channel signal B and extracts a modulated signal of common pilot channel signal B.
Channel estimation section 17 estimates (so-called “channel estimation”) the phase and amplitude of a propagation path using pilot symbols in the modulated signal of the individual channel signal output from despreading section 14. In the following explanations, the phase and amplitude of an estimated propagation path will be referred to as a “channel estimation value”.
Likewise, channel estimation section 18 performs a channel estimation using pilot symbols in the modulated signal of common pilot channel signal A output from despreading section 15 and channel estimation section 19 performs a channel estimation using pilot symbols in the modulated signal of common pilot channel signal B output from despreading section 16.
Demodulation section 20 demodulates the modulated signal of the individual channel signal output from despreading section 14 based on the channel estimation value output from channel estimation section 17.
Phase rotation control section 21 generates a phase rotation control signal that indicates the base station an amount of phase rotation based on a phase difference between common pilot channel signal A output from channel estimation section 18 and common pilot channel signal B output from channel estimation section 19.
Multiplexing section 22 multiplexes the transmission signal and the phase rotation control signal output from phase rotation control section 21. Modulation section 23 performs primary modulation processing such as QPSK on the output signal of multiplexing section 22. Spreading section 24 spreads the output signal of modulation section 23 by multiplying it by a specific spreading code. Transmission RF section 25 converts the frequency of the output signal of spreading section 24 to a radio frequency and transmits the signal by radio from antenna 11 via duplexer 12.
Then, a relationship between phase difference δ between individual channel signals and a channel estimation value estimated by channel estimation section 17 will be explained using FIG. 3A and FIG. 3B.
FIG. 3A shows channel estimation values when phase difference δ between individual channel signal A and individual channel signal B is −90°≦δ≦90°, while FIG. 3B shows channel estimation values when phase difference δ between individual channel signal A and individual channel signal B is 90°≦δ≦270°.
In FIG. 3A and FIG. 3B, channel estimation value β (n) is expressed as a synthesized vector of channel estimation value βa(n) of individual channel signal A and channel estimation value βb(n) of individual channel signal B. Moreover, a channel estimation value resulting from a synthesis of −βb(n) obtained by rotating βb(n) by 180° and βa(n) is expressed as β′(n).
The longer the vectors of channel estimation values β(n) and β′(n), the greater the reception power of the communication terminal becomes and the reception quality improves.
As shown in FIG. 3A, when phase difference δ between individual channel signal A and individual channel signal B is −90°≦δ≦90°, β(n) is greater than β′(n).
On the other hand, as shown in FIG. 3B, when phase difference δ between individual channel signal A and individual channel signal B is 90°≦δ≦270°, β′(n) is greater than β(n).
That is, when 90°≦δ≦270°, transmitting individual channel signal B rotated by 180° makes it possible to increase reception power at the communication terminal.
As shown above, in a radio communication system using transmission diversity, the reception quality can be improved by the communication terminal controlling the amount of phase rotation by carrying out channel estimations of common pilot channel signal A and common pilot channel signal B and the base station transmitting individual channel signal B by rotating its phase as appropriate based on the amount of phase rotation control and thereby increasing the reception power of the individual channel signals at the communication terminal.
However, when the base station rotates the phase of the individual channel signals for every slot as appropriate, the reception slots at the communication terminal become discontiguous, and therefore the conventional communication terminal above fails to average channel estimation values over a plurality of slots, having a problem of decreasing the reliability of channel estimation values compared to a case where transmission diversity is not used.